Fix for Issue 1416

src/ImageSharp/Processing/Processors/Normalization/GlobalHistogramEqualizationProcessor{TPixel}.cs

@@ -6,6 +6,7 @@
 using System.Numerics;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Threading;
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.PixelFormats;
@@ -51,7 +52,7 @@ protected override void OnFrameApply(ImageFrame<TPixel> source)
 
             using IMemoryOwner<int> histogramBuffer = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean);
 
-            // Build the histogram of the grayscale levels
+            // Build the histogram of the grayscale levels.
             var grayscaleOperation = new GrayscaleLevelsRowOperation(interest, histogramBuffer, source, this.LuminanceLevels);
             ParallelRowIterator.IterateRows(
                 this.Configuration,
@@ -114,7 +115,6 @@ public GrayscaleLevelsRowOperation(
 #endif
             public void Invoke(int y)
             {
-                ref int histogramBase = ref MemoryMarshal.GetReference(this.histogramBuffer.GetSpan());
                 ref TPixel pixelBase = ref MemoryMarshal.GetReference(this.source.GetPixelRowSpan(y));
                 int levels = this.luminanceLevels;
 
@@ -123,7 +123,8 @@ public void Invoke(int y)
                     // TODO: We should bulk convert here.
                     var vector = Unsafe.Add(ref pixelBase, x).ToVector4();
                     int luminance = ImageMaths.GetBT709Luminance(ref vector, levels);
-                    Unsafe.Add(ref histogramBase, luminance)++;
+                    ref int histogramAtLuminance = ref MemoryMarshal.GetReference(this.histogramBuffer.Slice(luminance));
+                    Interlocked.Increment(ref histogramAtLuminance);
                 }
             }
         }

tests/ImageSharp.Tests/Processing/Normalization/HistogramEqualizationTests.cs

@@ -17,7 +17,7 @@ public class HistogramEqualizationTests
         [Theory]
         [InlineData(256)]
         [InlineData(65536)]
-        public void HistogramEqualizationTest(int luminanceLevels)
+        public void GlobalHistogramEqualization_WithDifferentLumanceLevels(int luminanceLevels)
         {
             // Arrange
             var pixels = new byte[]
@@ -45,20 +45,21 @@ public void HistogramEqualizationTest(int luminanceLevels)
 
                 var expected = new byte[]
                 {
-                0,    12,   53,   32,  146,   53,  174,   53,
-                57,   32,   12,  227,  219,  202,   32,  154,
-                65,   85,   93,  239,  251,  227,   65,  158,
-                73,  146,  146,  247,  255,  235,  154,  130,
-                97,  166,  117,  231,  243,  210,  117,  117,
-                117, 190,   36,  190,  178,   93,   20,  170,
-                130, 202,   73,   20,   12,   53,   85,  194,
-                146, 206,  130,  117,   85,  166,  182,  215
+                    0,    12,   53,   32,  146,   53,  174,   53,
+                    57,   32,   12,  227,  219,  202,   32,  154,
+                    65,   85,   93,  239,  251,  227,   65,  158,
+                    73,  146,  146,  247,  255,  235,  154,  130,
+                    97,  166,  117,  231,  243,  210,  117,  117,
+                    117, 190,   36,  190,  178,   93,   20,  170,
+                    130, 202,   73,   20,   12,   53,   85,  194,
+                    146, 206,  130,  117,   85,  166,  182,  215
                 };
 
                 // Act
                 image.Mutate(x => x.HistogramEqualization(new HistogramEqualizationOptions
                 {
-                    LuminanceLevels = luminanceLevels
+                    LuminanceLevels = luminanceLevels,
+                    Method = HistogramEqualizationMethod.Global
                 }));
 
                 // Assert
@@ -75,6 +76,24 @@ public void HistogramEqualizationTest(int luminanceLevels)
             }
         }
 
+        [Theory]
+        [WithFile(TestImages.Jpeg.Baseline.HistogramEqImage, PixelTypes.Rgba32)]
+        public void GlobalHistogramEqualization_CompareToReferenceOutput<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage())
+            {
+                var options = new HistogramEqualizationOptions
+                {
+                    Method = HistogramEqualizationMethod.Global,
+                    LuminanceLevels = 256,
+                };
+                image.Mutate(x => x.HistogramEqualization(options));
+                image.DebugSave(provider);
+                image.CompareToReferenceOutput(ValidatorComparer, provider, extension: "png");
+            }
+        }
+
         [Theory]
         [WithFile(TestImages.Jpeg.Baseline.LowContrast, PixelTypes.Rgba32)]
         public void Adaptive_SlidingWindow_15Tiles_WithClipping<TPixel>(TestImageProvider<TPixel> provider)

tests/ImageSharp.Tests/TestImages.cs

@@ -196,6 +196,7 @@ public static class Bad
                 public const string YcckSubsample1222 = "Jpg/baseline/ycck-subsample-1222.jpg";
                 public const string Iptc = "Jpg/baseline/iptc.jpg";
                 public const string App13WithEmptyIptc = "Jpg/baseline/iptc-psAPP13-wIPTCempty.jpg";
+                public const string HistogramEqImage = "Jpg/baseline/640px-Unequalized_Hawkes_Bay_NZ.jpg";
 
                 public static readonly string[] All =
                 {